Corona voltage supply for use in electrostatic copier

ABSTRACT

A power supply is disclosed which provides a plurality of highvoltage DC outputs suitable for driving the corona voltage units located at the charging, developing, and transferring stations of an electrophotographic copying apparatus. Each of the outputs is electrically isolated from the others by means of individual rectifying and isolating diodes to prevent voltage fluctuations appearing at any one of the outputs from being communicated to and adversely affecting the voltage at any of the other outputs.

United States Patent Inventor Appl. No.

Filed Patented Assignee Priority Detlef Sehaffer Wetzlar, Germany 768,263

Oct. 17, 1968 July 6, 1971 The Singer Company Nov. 22, 1967 Germany CORONA VOLTAGE SUPPLY FOR USE IN ELECTROSTATIC COPIER 10 Claims, 2 Drawing Figs.

US. Cl 307/31, 250/495 Int. Cl H02j 1/00 Field 01 Search ..250/49.561,

[56 1 References Cited UNITED STATES PATENTS 3,308,344 3/1967 Smith et al 250/495 (60) Primary ExaminerR0bert K. Schaefer Assistant Examiner-H. J. Hohauser Attorneys-Charles R. Lepchinsky and Jay M. Cantor ABSTRACT: A power supply is disclosed which provides a plurality of high-voltage DC outputs suitable for driving the corona voltage units located at the charging, developing, and transferring stations of an electrophotographic copying apparatus. Each of the outputs is electrically isolated from the others by means of individual rectifying and isolating diodes to prevent voltage fluctuations appearing at any one of the outputs from being communicated to and adversely affecting the voltage at any of the other outputs.

PATENTED JUL BIB?! SHEET 2 OF 2 CORONA VOLTAGE SUPPLY FOR USE IN ELECTROSTATIC COPIER BACKGROUND OF THE INVENTION This invention relates to an electrophotographic copying apparatus in which a copy of an original document is produced by an electrophotographic process. In copending U.S. application, Ser. No. 820,531, filed Apr. 30, 1969 there is disclosed an electrophotographic copying apparatus in which a continuous translucent web or tape of material having a photoconductive layer is unwound from a supply reel and then led in succession past a charging station, an exposing station, a developing station, a transfer station, and a cleaning station, to a takeup reel. At the charging station, electrostatic surface charges are distributed on the photoconductive surface of the tape by a corona voltage unit. The charged tape is then led past the exposing station, where a latent electrostatic image of the original document to be copied is formed on the photoconductive surface of the tape by a reflex exposure process. In such a process, the original is placed against the photoconductive side of the translucent tape while, simultaneously, light is irradiated onto the other side of the tape. The

light which passes through the tape is reflected by the original back to the photoconductive layer at different intensities: the dark areas of the original-corresponding to the image desired to be copied in a positive original-absorb most of the light; the light areas of the original-corresponding to the desired nonimage areas in a positive original-reflect most of the light. Those surface charges lying in the path of the reflected light are dissipated, while the remaining surface charges are unaffected. The result is a latent electrostatic image of the original on the photoconductive surface of the tape.

After exposure, the tape is led past the developing station where an electrostatically charged developing powder or toner is sprinkled on the tape surface. Because the charged powder particles are of opposite sign to those charges on the I tape surface, they adhere to those charges to form a powder image on the tape surface. Moreover, since there are ideally no charges on the nonimage regions of the tape surface, the

toner will ordinarily not adhere to the surface at these regions.

The tape next is conducted past the transfer station where the powder image is transferred to the copy surface-ordinarily a sheet of paperby pressing the two surfaces together in the presence of a corona voltage unit which produces a potential of opposite sign to that of the toner particles, thereby attracting the particles toward the copy surface. After transfer, any toner particles remaining on the tape are removed from the photoconductive surface at the cleaning station, for example, by brushing. The tape is then wound about the takeup reel.

It frequently happens that not all of the electrostatic charges present on a nonimage area of the photoconductive surface of the charged tape are dissipated during exposure. Unless dissipated or otherwise compensated for, these charges will attract toner during the developing step, which, in turn, will be transferred to the copy surface during the transfer step and produce a copy with gray areas or dark spots instead of a desirable white background. To preclude this undesirable result, a corona voltage unit which produces a potential having the same sign as that of the toner charges is placed opposite the developing station. This arrangement has the effect of causing toner to adhere only to those areas of the photoconductive surface having a large charge concentration, which areas correspond to the unexposed areas of the photoconduc tive surface, that is, the desired image area.

In the specific embodiment disclosed in the above-mentioned copending U.S. application, which employs negative charges on the photoconductive surface, a power supply is required which will supply a high negative DC potential to the corona voltage units at the charging and transfer stations, and a high positive DC potential to the corona voltage unit which is opposite the developing station. In order to produce uniform copies of good quality, it is imperative that all corona voltage units operate at stable potential levels. If the knowntype of power supply heretofore employed in an electrophotographic'copying apparatus is used, however, this is not the case, for as the tape moves into the field region of each unit in a fully charged or partially charged state, it alters the nature and value of the field with a resulting change in potential of the unit. For example, after leaving the developer station, the tape bearing the charged toner particles passes by the corona voltage unit at the transfer station. The proximity of these tape-bome particles to the unit alters the value of the potential at this unit. Since this corona voltage unit is connected to the same voltage supply output as the corona voltage unit which is used to charge the photoconductive surface of the tape, this fluctuation in potential is communicated to the charging station. Thus, the passage of the charged tape through the transfer station alters the potential of the corona voltage charging unit, the amount of fluctuation depending, among other things, upon the charge configuration on the tape. Since the charge configuration on the tape varies widely along its lengths, the charging station potential also varies widely over a given period of time. This results in a nonuniform charge distribution on the unexposed tape which seriously affects the quality of the final copy produced. 1

SUMMARY OF THE INVENTION According to the applicants invention, there is provided a power supply suitable for use in an electrophotographic copying apparatus as a corona voltage supply means and which furnishes a plurality of high voltage outputs, each of which is electrically isolated from the others in order to prevent variations in the potential level at any one output from influencing the potential level at any of the other outputs. In a further aspect of the invention, two of the outputs providing a high voltage potential of a first polarity are connected to the charging and transfer stations, respectively, of the copying apparatus, while another output providing a high voltage of opposite polarity is connected to the corona voltage unit op-- posite the developing station in the apparatus. Each output is electrically isolated from the others by means of a separate diode with one side of each of the diodes being connected to one side of the secondary circuit of the power supply transformer. A separate capacitor is connected between each output and a reference potential to smooth fluctuations in the rectified voltage appearing at each output.

For a fuller understanding of the nature and advantages of the invention, reference should be had to the following detailed description, taken in conjunction with the accompanying drawings wherein like reference characters designate like or similar elements throughout the various views and in which:

FIG. I shows a schematic sectional elevational view of an electrophotographic copying apparatus for implementing the invention; and I FIG. 2 is an electronic schematic diagram of a power supply embodying the invention.

The electrophotographic copying apparatus of FIG. I, which is disclosed in my copending U.S. application, Ser. No. 820,531, filed Apr. 30, I969, comprises a flexible tape or web 10, having a suitable photoconductive insulating layer-for example, zinc oxidedisposed on the outer surface thereof and which is supplied from the reel 12, passing over rollers 13 and 14. Adjacent roller 14 is positioned a corona discharge electrode 15 connected to one of the negative outputs of a power supply (as shown in FIG. 2) which supplies a high DC potential for distributing a negative electrical charge over the photoconductive surface of the tape 10 in the usual manner. Immediately upon charging the surface of the tape, an original to be reproduced, indicated by arrow 22, is fed by means of rollers 16 and 17 through suitable guide ways into engagement with the charged photoconductive surface of the tape 10. Thereafter, the original moves with the tape 10 over a reflex exposure roller 18 while light from a source 19 is reflected by a mirror 19' onto the transparent or translucent tape. The light passing through tape It) strikes the surface of the original and is reflected back with differential intensity: the nondarkened portions of the original surface reflect substantially all the incident light, while the dark areas of the original absorb substantially all of the light. The strongly reflected light which strikes the photoconductive surface of tape dissipates the electrostatic charges remaining on the tape surface. The result is a latent electrostatic image of the original on the photoconductive surface of the tape. During this process, the original and the tape 10 pass the exposure station at the same linear speed with the message side of the original in contact with the photoconductive surface. After the original has been completely exposed, it is separated from the tape 10 by a separator plate 23 and is ejected by means of rollers 20 and 21.

Upon separation of the original sheet from the tape 10, the tape continues in its path of travel and, between supporting rollers 26 and 28, the latent electrostatic image formed on the surface of the tape is dusted with developer powder or toner of a positive polarity by means of a magnetic developer brush 27, rendering the latent image visible. The magnetic developer brush 27 may comprise a magnet with a mass of iron filings or ferromagnetic powder attached to it by magnetic attraction. The chain-like arrangements of magnetic particles simulate the fibers of a brush. The developer powder may be any suitable substance, such as a resin toner, which is so related to the magnetic particles on the triboelectric scale, that, when applied to the brush, will cling to the magnetic fibers by triboelectric attraction. A corona voltage unit 25, which is connected to the positive'DC output of the power supply (as shown in FIG. 2), opposes the application of the positively charged developing powder to the tape surface. This prevents the developing powder from adhering to any charges which remain on the nonimage areas of the surface of the tape due to incomplete dissipation during the exposure step. The developed image area of the tape .10 then passes over roller 28 located at the vertex of an acute angle defining the course of travel of the tape between supporting roller 26 and over a supporting roller 29. As the tape 10 bearing the developed image passes between rollers 28 and 29, a transfer unit 30, which is disposed above and slightly spaced from the tape between rollers'28 and 29 and which is connected to the other negative output from the power supply (as shown in FIG. 2), effects the transfer of the developed image onto a sheet of copy paper by attracting the positively charged developer powder to the copy surface. This unit is similar in nature to the corona discharge electrode 15. While corona voltage units 15, 25 and 30 are illustrated in the figures as having single electrodes, it is clear that other suitable types, such as units having three electrodes, can be employed as full equivalents.

To provide the copy paper, sheets of ordinary paper are separated, sheet by sheet, from a stack of sheets 31 by a separator roller 32, and upon separation each sheet of copy paper is moved by feed rollers 33 and 34 into engagement with tape 10 in timed relation to the movement of the developed image area on the tape and in a plane parallel to the run of the tape between rollers 28 and 29. The movement of the copy sheet is synchronous with that of the tape. As the sheet of copy paper and the developed image area of the carrier tape 10 move together beneath transfer unit 30, the powder image is attracted from the surface of the tape by the potential on the electrode of transfer unit 30 and transferred to the surface' of the copy paper. Following the transfer of the powder image from the photoconductive surface of the tape 10 to the sheet of copy paper, the sheet is grasped by a cylinder 38, prior to its release by feed rollers 33, 34 for passage through a fixing'station 39 where the powder image is fused, for example, by heating the copy paper. The finished copy is thereafter ejected by means of a pair of rollers 40 and 41. in order to grip each copy sheet, the wall of the cylinder 38 is provided with a series of apertures 42 spaced axially and circumferentially of the cylinder, certain of the apertures 42 being open to a subatmospheric pressure supplied within the cylinder during its rotation. Only those apertures 42 over approximately of the cylinders circumference are open to the subatmospheric pressure supplied within the cylinder during its rotation. The other apertures 42 are closed so that as the leading end of a copy sheet approaches the lower surface of the cylinder 38, the vacuum within the cylinder secures the copy sheet to the surface thereof, releasing it at the top for movement into the nip of ejector rollers 40, 41. The rotational speed of cylinder 38 is identical to that of the surface speed of tape 10.

Continuing in its path of movement, the image area of the tape 10 passes through a cleaning station 43, between a cleaning brush 44 and a roller 45, thence over rollers 46, 47 and 48 to a takeup reel 49. As the photoconductive surface of the tape 10 moves into contact with the cleaning brush 44, the residual developer powder thereon is brushed from the tape and the residue is removed by vacuum means (not shown). Once the tape 10 is completely wound on takeup reel 49, it may be immediately rewound on supply 12 for reuse. The time required to completely wind tape 10 onto takeup reel 49 and to again rewind the tape onto supply reel 12 is sufficient to cause any residual charge on the photoconductive surface of .the tape to be completely dissipated so that the tape is again ready for reuse.

FIG. 2 shows a power supply suitable for furnishing corona voltage units 15, 25 and 30 with their respective operating voltages. A transformer 50 has two input terminals, 52 and 54, which can be connected to any suitable AC source-for example, l 10 volts AC. One side 56 of the transformer secondary is connected to ground. Rectifying and isolating diodes 60, 62 and 64 each have one of their elements connected to the other side 58 of the secondary circuit, the element being chosen in accordance with the desired polarity of the rectified output voltage. Diode 60, for example, has its anode connected to the side 58 in order to produce a positive output of several thousand volts DC to drive corona voltage unit 25. Diodes 62 and 64, on the other hand, each have their cathodes connected to side 58 in order to provide a negative output of several thousand volts DC for driving corona voltage units 15 and Capacitors 66, 68 and 70 are connected between the outputs of diodes 60, 62 and 64, respectively, and ground to smooth out any fluctuations in the output voltages due to rectification. The choice of Specific circuit elements employed depends on the circuit parameters desired, and is well within the competence of one of ordinary skill in the art.

An important feature of the power supply of FIG. 2 is the use of separate diodes for each output furnished. This prevents any voltagefluctuations appearing in any of the outputs from being communicated to any of the other outputs. For example, during the transfer process, the magnitude of the negative potential on the electrode of transfer unit 30 will vary in accordance with the amount of developer powder on the surface of the tape 10 which is in close proximity to this unit. Due to the presence of diode 62, however, these fluctuations in magnitude will not be communicated to the electrode of charging unit 15. Since the potential of charging unit 15 is unaffected by these fluctuations in magnitude, surface charges will continue to be distributed on the tape 10 at a constant rate, thereby ensuring a uniform surface charge distribution thereon. Likewise, any fluctuation in the potential on the electrdde of charging unit 15 will not be communicated to the electrode of transfer unit 30 because of the presence of diode 64, thereby ensuring that the transfer potential remainsconstant so that the developer powder will be transferred to the copy surface at a constant rate.

While the invention has been disclosed and described in terms of an electrophotographic copying system which employs negative electrostatic charges distributed on a photoconductive surface, it is clear that the same principles apply to a system utilizing positive charges distributed on a photoconductive surface, in which case the power supply would be designed to produce two positive outputs and one negative output. it is to be understood that numerous other arrangements and modifications, as well as other applications, may be devised by one skilled in the ant without departing from the spirit and scope of this invention.

What I claim is:

1. For use in an electrostatic copier having a corona charging unit, a corona developer unit, and a corona transfer unit, a power supply for supplying an isolated potential to each one of said corona units, said power supply comprising a transformer having a primary circuit and a secondary circuit, said secondary circuit having one side connected to a reference potential, and means for electrically isolating each one of said corona units from remaining ones of said corona units to prevent voltage fluctuations appearing at any one of said corona units from being communicated to any of the other said corona units through said power supply, said isolating means including a plurality of unidirectional conducting devices, each having an input terminal and an output terminal, the input terminal of each of said plurality of unidirectional conducting devices being directly connected to the other side of said secondary circuit at a common junction, the output terminals of each of said plurality of unidirectional conducting devices being directly connected to a separate one of said corona units.

2. The apparatus of claim 1' wherein at least one of said unidirectional conducting devices is poled in a direction opposite that of the remaining ones of said unidirectional conducting devices.

3. The device of claim 1 wherein each of said unidirectional conductingdevices comprises a diode having an anode and a cathode.

4. The apparatus of claim -3 wherein said diodes are poled to provide a high negative potential to said corona charging unit andsaid corona transfer unit, and a high positive potential to said corona developer unit.

5. The apparatus of claim 4 wherein said cathode of each of said diodes poled to provide a high negative potential is directly connected to said common junction and said anode of each of said diodes poled to provide a high positive potential is directly connected to said common junction.

6. The apparatus of claim 1 further including a capacitor coupled between said output terminal of at least one of said plurality of unidirectional conducting devices and said reference potential.

7. The apparatus of claim 1 further including a plurality of capacitors coupled between said reference potential and said plurality of output terminals, each one of said plurality of capacitors being connected to a different one of said output terminals.

8. A power supply for providing isolated high potential to a corona charging unit, a corona developer unit, and a corona transfer unit in an electrostatic copier, said power supply comprising a transformer having a primary circuit and a secondary circuit, a plurality of output terminals, each of said plurality of output terminals adapted to be connected to a different one of said corona units and means for electrically isolating each of said output terminals from remaining ones of said output terminals, said last-named means including a plurality of unidirectional conducting devices each having an input terminal and an output terminal, said input terminal of each of said unidirectional conducting devices being directly connected to a first side of said secondary circuit at a common junction, said output terminal of each of said unidirectional conducting devices being directly connected to a different one of said power supply output terminals, at least one of said unidirectional conducting devices being poled in a direction opposite that of the remaining ones of said unidirectional conducting devices, the other side of said secondary circuit being connected to a reference potential.

9. The apparatus of claim 8 further including a plurality of capacitors coupled between said output terminal of said unidirectional conducting devices and said reference potential, each one of said plurality of capacitors being coupled to a different one of said out ut terminals.

The apparatus 0 claim 8 wherein each one of said unidirectional conducting devices comprises a diode having an anode and a cathode. 

1. For use in an electrostatic copier having a corona charging unit, a corona developer unit, and a corona transfer unit, a power supply for supplying an isolated potential to each one of said corona units, said power supply comprising a transformer having a primary circuit and a secondary circuit, said secondary circuit having one side connected to a reference potential, and means for electrically isolating each one of said corona units from remaining ones of said corona units to prevent voltage fluctuations appearing at any one of said corona units from being communicated to any of the other said corona units through said power supply, said isolating means including a plurality of unidirectional conducting devices, each having an input terminal and an output terminal, the input terminal of each of said plurality of unidirectional conducting devices being directly connected to the other side of said secondary circuit at a common junction, the output terminals of each of said plurality of unidirectional conducting devices being directly connected to a separate one of said corona units.
 2. The apparatus of claim 1 wherein at least one of said unidirectional conducting devices is poled in a direction opposite that of the remaining ones of said unidirectional conducting devices.
 3. The device of claim 1 wherein each of said unidirectional conducting devices comprises a diode having an anode and a cathode.
 4. The apparatus of claim 3 wherein said diodes are poled to provide a high negative potential to said corona charging unit and said corona transfer unit, and a high positive potential to said corona developer unit.
 5. The apparatus of claim 4 wherein said cathode of each of said diodes poled to provide a high negative potential is directly connected to said common junction and said anode of each of said diodes poled to provide a high positive potential is directly connected to said common junction.
 6. The apparatus of claim 1 further including a capacitor coupled between said output terminal of at least one of said plurality of unidirectional conducting devices and said reference potential.
 7. The apparatus of claim 1 further including a plurality of capacitors coupled between said reference potential and said plurality of output terminals, each one of said plurality of capacitors being connected to a different one of said output terminals.
 8. A power supply for providing isolated high potential to a corona charging unit, a corona developer unit, and a corona transfer unit in an electrostatic copier, said power supply comprising a transformer having a primary circuit and a secondary circuit, a plurality of output terminals, each of said plurality of output terminals adapted to be connected to a different one of said corona units and means for electrically isolating each of said output terminals from remaining ones of said output terminals, said last-named means including a plurality of unidirectional conducting devices each having an input terminal and an output terminal, said input terminal of each of said unidirectional conducting devices being directly connected to a first side of said secondary circuit at a common junction, said output terminal of each of said unidirectional conducting devices being directly connected to a different one of said power supply output terminals, at least one of said unidirectional conducting devices being poled in a direction opposite that of the remaining ones of said unidirectional conducting devices, the other side of said secondary circuit being connected to a reference potential.
 9. The apparatus of claim 8 further including a plurality of capacitors coupled between said output terminal of said unidirectional conducting devices and said reference potential, each one of said plurality of capacitors being coupled to a different one of said output terminals.
 10. The apparatus of claim 8 wherein each one of said unidirectional conducting devices comprises a diode having an anode and a cathode. 